Add Shell sort implementation with tests

[williamfiset]

Summary

Adds a Shell sort implementation to the sorting package, following the repo's existing sorting conventions.

Shell sort is implemented as a gapped insertion sort using the original Shell gap sequence (n/2, n/4, …, 1). The large-gap passes move out-of-place elements most of the way toward their final position in big jumps, so the final gap-1 pass (an ordinary insertion sort) has very little left to do.

Changes

• ShellSort.java — implements the shared InplaceSort interface, with a teaching-style doc comment explaining how and why the algorithm works, its time/space complexity, and a runnable main method (matches the style of InsertionSort/TimSort).
• ShellSortTest.java — JUnit 5 + Truth tests mirroring the existing sort tests: empty, single, already-sorted, reverse-sorted, duplicates, all-same, negatives, mixed sign, two-element, plus a randomized pass over sizes 0–499 compared against Arrays.sort.
• SortingTest.java — registers SHELL_SORT in the shared enum/EnumSet so it's also covered by the cross-algorithm property tests.
• Bazel BUILD files — add the ShellSort java_binary target and the ShellSortTest java_test target.
• README.md — add a Shell sort entry to the sorting algorithms list.

Complexity

• Time: O(n²) worst case with the Shell sequence, ~O(n^1.5) average, O(n log n) best case (already sorted).
• Space: O(1) — sorts in place.

Testing

• bazel test //src/test/java/com/williamfiset/algorithms/sorting:ShellSortTest //src/test/java/com/williamfiset/algorithms/sorting:SortingTest → both PASSED.
• bazel run //src/main/java/com/williamfiset/algorithms/sorting:ShellSort prints [-13, 2, 3, 4, 4, 6, 8, 10] as documented.

Note: the new file uses @author Sculptor in its header, consistent with the recently added TimSort using a non-personal author tag. Happy to change it if a different attribution is preferred.